1. Field of the Invention
The present invention relates to an electronic scanning radar apparatus, a received wave direction estimating method, and a received wave direction estimating program used therein, which can detect a target using a reflected wave from a target in response to a transmitted wave and which can be suitably used for a vehicle.
2. Background Art
Electronic scanning type radars such as an FMCW (Frequency Modulated Continuous Wave) radar, a multi-frequency CW (Continuous Wave) radar, and a pulse radar have been known as an on-board radar for some time.
In the radars, an arrival wave direction estimating method of an array antenna is used as a technique of detecting the direction of an arrival wave (or a received wave) from a target (a reflecting object).
As such an arrival wave direction estimating method, high-resolution (high-accuracy) algorithms such as an AR spectrum estimating method (FIG. 34) and a MUSIC (MUltiple Signal Classification) method which can achieve a high resolution without increasing the number of channels of a receiving antenna have been used (see JP-A-2006-275840 (Patent Document 1), JP-A-2009-156582 (Patent Document 2), and JP-A-2009-162688 (Patent Document 3)). The AR spectrum estimating method may also be called a maximum entropy method (MEM) or a linear prediction method.
When the direction of an arrival wave from a target (a reflecting object) is estimated using such algorithms, the estimation process is performed after input data (data in which a noise component is mixed into a complex sinusoidal wave) expressed by a complex number is transformed into a matrix format such as a correlation matrix.
In a high-resolution arrival wave direction estimating method used in an on-board radar, a correlation matrix in a previous control cycle is stored and is subjected to an averaging process (or an adding process) with a correlation matrix in the present control cycle and then a direction estimating process is performed, in order to suppress a noise component and to improve estimation accuracy. This correlation matrix averaging process can provide a great effect for improvement in azimuth detection accuracy (angular accuracy or resolution) in an on-board radar having a small number of channels (Patent Document 2).
In the high-resolution algorithms, it is necessary to perform the estimation after setting an appropriate number of arrival waves (received waves) (the model order in the case of the AR spectrum estimating method) (Patent Document 3).